In the manufacture of electronic devices, such as computers, printed circuit boards are widely used to support discrete electronic components and to provide the electrical circuitry between the components. Commercial electronic computers have become much more powerful since the introduction thereof, yet these have been reduced in physical dimensions from room size to notebook size. As this size has decreased and the number of interconnections due to more powerful logic has increased, the printed circuit boards used therein have become denser and more complex. Today's printed circuit boards can be extremely dense, with very small geometries and with many layers.
Typically, printed circuit boards have at least one central core, typically a dielectric layer, such as a composite of fiber glass and a thermosetting resin such as an epoxy resin, which core has applied on at least one surface thereof a layer of conductive material such as copper. The layer or layers of conductive material are etched or otherwise processed to provide circuits of predetermined geometrical configuration. Several such cores may then be laminated to form a multilayered structure (a printed circuit board) having metal circuitry sandwiched between dielectric layers.
The dielectric material widely used today contains a composition of epoxy resins impregnated onto glass or fiber-glass reinforcing material. The dielectric layers within these circuit boards exhibit the color of the epoxy, typically white or yellow. Printed circuit boards with dielectric surfaces and layers having a white or yellow color afford low optical contrast when compared with the circuitry and make automated optical assembly pick and place operations (where components are positioned on and eventually coupled to the circuitry) very difficult.
Additionally, in the likely event that the printed circuit boards made from these dielectric layers require inspection, low optical contrast between the dielectric material and circuitry on any one layer of the printed circuit board also makes inspection difficult and inefficient. For example, circuitry defects adjacent the dielectric material on a printed circuit board with low optical contrast are difficult to optically inspect and make the process of inspecting very time consuming.
Electrical techniques to detect such circuit defects are also not effective because high electrical conductivity metals are used in circuitizing printed circuit boards. Detecting a change in current flow through a highly conductive metal circuit having a small defect is very difficult, if not impossible, without very sophisticated techniques.
In application Ser. No. 09/025,513, a high optical contrast and UV fluorescing composition is described for use in dielectric layers of printed circuit boards. The invention described herein provides a significant improvement over Ser. No. 09/025,513. An improved substrate having a dielectric layer, that does not include a reinforcing material, with high optical contrast and UV fluorescing characteristics has been developed. It is believed that such a substrate and method of making the substrate, will constitute a significant advancement in the art.